A scanning electron microscope is an instrument which focuses a primary electron beam emitted from an electron gun on a sample through a magnetic field lens, scans the sample with the primary electron beam through a magnetic field deflector or an electric field deflector, and detects secondary charged particles (for example, secondary electrons or scanning transmission electrons) from the sample, thereby obtaining an enlarged image of the sample. The observation magnification of the enlarged image of the sample is defined by the ratio between the scanning width of the primary electron beam on the sample surface and the display width of the enlarged image formed by the secondary charged particles obtained from the scanned area. The scanning width of the primary electron beam on the sample can be arbitrarily changed by the deflector. Therefore, if the display width of the enlarged image is constant, the observation magnification decreases by widening the scanning area of the primary electron beam on the sample, and the observation magnification increases by narrowing the scanning area. Hereinafter, for the sake of simplicity, the following description will be made on the assumption that the magnification value in this specification is defined with the display width of the enlarged image of 100 mm which is close to the generally used value. In this case, a magnification of 10,000× indicates a state in which a sample image of a 10-μm region is displayed on an enlarged image with a width of 100 mm.
Recently, samples to be observed with a scanning electron microscope have been so miniaturized, and the observation at a magnification of 1,000,000× or more (display region of sample: 100 nm or less), which has hardly been practiced, has been required. In addition, in order to measure the dimensions of a sample structure, highly accurate and reliable dimensional calibration (magnification calibration) is required in this magnification range. The dimensional calibration requires accurate dimensional measurement at a magnification higher than that at the observation.
Conventionally, in the execution of the accurate dimensional calibration, by the use of a microscale having known dimensions, the dimensions of several pitches thereof are measured, and the resultant value is used as a true value. In addition, when a secondary charged particle is a scanning transmission electron, as disclosed in the Patent Document 1, a lattice image is obtained by using a thin single crystal sample having a known crystalline structure (lattice spacing), and dimensional calibration is performed by using the image.